spi: Add "spi:" prefix to modalias attribute of spi devices

The modalias attribute of spi devices doesn't have the "spi:" prefix
that is used in the UEVENT and in spi device drivers.  This patch adds
the prefix so the modprobe can correctly match modules to devices.

Reported-by: David Daney <david.daney@cavium.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>